It depends on the personal preference of the person you are making the scarf for, and how that person wants to wear it, I think. My friend and I were discussing this recently. Neither of us are very tall and we seem to look overwhelmed by a wide long scarf. It looks all too much. So we prefer narrow scarves. Another friend is tall and very slim and can "carry off" a long scarf whether it be wide or narrow. My husband prefers his scarves wider and rather likes them long enough to double and loop thru.
Some people say that a good rule is to make the scarf as long as the person is tall. For charity knitting I do not make them too long but with a bit of width, a long trailing scarf could be a problem to a homeless person.